Power line communication (PLC), also called Broadband over Power Lines (BPL), is a technology that uses electrical power networks to transmit data and voice signals in providing online services. Recently, high-speed radio frequency (RF) data transmission over medium voltage (MV) power lines has become feasible. A variety of PLC-based broadband services, such as high-speed Internet connection, video on demand, electric grid modernization and telecommuting, are being developed.
In PLC, the communication signals coexist, but do not interact, with the standard 50 or 60 Hz alternating current (AC). The signals travel along the power lines and pass through or around utility transformers to service subscribers' homes and businesses, as well as to utility controlling and monitoring equipment.
A subscriber, or a utility monitoring device such as a meter, uses a modem to extract the communication signals from the power lines and to inject such communication signals into the power line. The modem is connected to the power line via a coupling system. For a power line to accommodate high-speed communications, the coupling system must be designed so that it can efficiently couple radio-frequency signals to and from the power network. Such a coupling system must not compromise the performance of the existing power system. It must be able to shield various low-voltage electronic components of the RF modem from high voltage, steady state and transient electrical power, and it must tolerate the harsh physical and electrical environment associated with MV overhead power lines, including exposure to high voltage surges associated with lightning strikes and switching surges. In practice, such a coupling system must be inexpensive, compact, environmentally acceptable, essentially maintenance free, safe and easy to install.
Various capacitive or inductive coupling systems exist. One example of the coupling systems is a so-called converter described in Sanderson (U.S. Pat. No. 5,864,284). The converter comprises a lightning arrester (a shunt device common to power utility operations) and a signaling device for coupling an RF signal. The signaling device comprises an RF modem coupled to an RF transformer. A high-voltage terminal of the arrester is connected to the power cable and a low-voltage terminal of the arrester is connected to a terminal of the transformer. Another terminal of the transformer is connected to a neutral node (earth ground). The low-voltage terminal of the arrester is also directly connected to the neutral node via a piece of grounding cable.
During installation of the coupling system, one or more ferrite cores are typically placed around the grounding cable in order to increase the RF impedance between the low-voltage terminal of the arrester and the neutral node. This kind of installation normally involves field configuration to modify an existing arrester installation by way of stacking ferrite cores around the grounding cable, which requires trained personnel and test equipment.
The present invention is directed to an RF signal coupling system that avoids the need for field configuration and which has other desirable properties, including the use of a lightning arrester in a manner that avoids the firing of the arrester under fault conditions, and the ability of the coupling system to conduct a fault to earth ground without the need for a parallel path to earth ground.